1. Field of the Invention
The present invention relates to an optical disk in which a laser beam is irradiated from a light transmitting layer side provided on a substrate to record and reproduce information, particularly relates to the optical disk in which a protective layer for the light transmitting layer is provided and a method of manufacturing the same.
2. Description of the Related Art
Recently, an optical recording medium such as an optical disk is becoming commonplace, for example, in DVD (Digital Versatile Disk) or DVD-R for a DVD apparatus, a laser beam is irradiated from a backside of a substrate to record and reproduce information. However, in the next-generation DVD apparatus or the like, recording and the reproducing are performed in such a manner that a recording layer is formed on the face of the substrate, a light transmitting layer is provided on the recording layer, and the laser beam is irradiated from a frontside of the disk.
That is, in the optical disk, the recording layer (a deposited layer which directly involves the recording, erasing, and reproducing characteristics of a phase change type of optical disk such as a reflecting film, a dielectric material film, and a phase change film) is formed on a transparent plastic substrate in which a guide groove is formed by an injection molding method or the substrate made by a so-call 2P method, and the light transmitting layer (a bonding film made of ultraviolet cured resin and a transparent sheet) is formed on the recording layer. Usually, the recording layer is formed by a sputtering method, and the light transmitting layer is formed in such a manner that resin liquid is applied on the substrate and the recording layer with a spin coat method and is hardened, and the transparent sheet is fixed.
However, when the light transmitting layer is formed up to an outermost peripheral end of the substrate, external force of handling causes a problem such as easy separation. The reason is as follows. That is, it is necessary that the light transmitting layer is larger than the recording layer in order to protect the recording layer. The light transmitting layer has a thickness of about 100 μm and requires to be covered with the transparent sheet having excellent evenness, because it is necessary that birefringence is decreased and the thickness is uniformed on the entire periphery surface of the substrate. However, in the case of adopting the above-described structure, since the outermost peripheral end portion of the transparent sheet constituting the light transmitting layer is as thick as 100 μm, it is easy to generate a step at the outermost peripheral end portion of the substrate. In some cases, the transparent sheet is separated by getting hitched during the handling, so that the recording layer has a possibility of being broken.
With reference to the method of manufacturing an optical disk, in Japanese Patent. Application KOKAI Publication No. 5-81701 which is a reference disclosing the technique forming the protective layer for protecting a given layer, there is disclosed a technique in which the protective layer and the resin layer are formed on a metal reflecting layer in order to prevent the metal reflecting layer from being separated from the substrate. In the optical disk, the laser beam is irradiated from the backside of the substrate to the recording layer and to record and reproduce the information.
However, since the optical disk shown by the reference is not the next-generation optical disk, the laser beam is irradiated from the backside of the substrate to the recording layer. Accordingly, unlike the optical disk according to one embodiment of the invention, the technique in which the light transmitting layer including the transparent sheet and a bonding layer is bonded to the substrate and is protected with sufficient strength, while transparency is retained, is not disclosed. Therefore, there is a problem that the light transmitting layer of the optical disk, in which the recording and the reproducing are performed by irradiating the laser beam from the frontside of the optical disk, can not be bonded and protected with the sufficient strength.
That is, according to the conventional manufacturing method, in the optical disk in which the recording and the reproducing are performed by irradiating the laser beam from the frontside of the optical disk, there is the problem that, while transparency is retained, the light transmitting layer including the transparent sheet and the bonding layer can not be bonded onto the substrate and protected with the sufficient strength.